ArmaLite AR-15
The AR-15 is a modular weapon system, primarily manufactured by Colt and several other companies. It is the basis for the M16 series of service rifles currently in use by the U.S. military, and for many semi-automatic rifles currently manufactured by several companies. Design Details The AR-15 is a lightweight, magazine-fed, gas-operated, air-cooled, shoulder-fired rifle. Civilian-manufactured models generally utilize the semi-automatic fire control group - the selector switch has options for SAFE and FIRE. These weapons are found with a modified hammer assembly, a modified disconnector, a modified trigger assembly, an unmilled shelf in the lower receiver, and they lack an auto sear. On the hammer, there is no notch (often called a 'J-hook') for the auto sear to catch when the carrier cycles back. The selector switch allows the sear on the trigger assembly to catch the hammer as the bolt carrier cycles rearward, preventing the hammer from re-engaging the firing pin and firing another round until the trigger is pulled again. To further prevent conversion to full auto, some manufacturers use bolt carriers modified to prevent engagement with an auto sear, if one were to be installed; approximately 1.25 inches of steel is milled off the rear part of the carrier, preventing the auto sear from releasing the hammer to fire again. Rifles manufactured for export or military sales have the automatic fire control group; the selector switch has options for SAFE, FIRE, and BURST (or AUTO). These weapons are found with the M16 hammer assembly, the M16 disconnector, and a trigger assembly that allows for the auto sear to function as intended in conjunction with the auto selector switch. The M16 lower receiver has a section of its shelf milled lower to accommodate the auto sear, and an additional hole (the sear hole) is drilled into the lower receiver to hold the sear in place. Bolt Carrier Assembly The bolt carrier assembly consists of the bolt, a firing pin, a retaining pin to hold the firing pin in its channel inside the bolt, a cam pin to secure the bolt as it locks and unlocks when the weapon cycles, and a gas key, which is attached to the top of the bolt carrier itself via two Allen screws (hex screws). The gas key should be properly staked, deforming the metal enough to prevent the screws from backing out during operation of the weapon. The screws should not be secured via Loc-tite; heat will break down the loc-tite and the screws will back out with continued operation. The bolt and bolt carrier should be composed of 4150 CMV steel for maximum durability, and the bolt should be shot peened, tested with a proof load, and MPI tested. Barrel and Bore In adherence to the TDP (Technical Data Package), the barrel should be hammer forged from 4150 CMV (Chrome-moly Vanadium) steel. This allows the barrel to last a bit longer under the stresses of full auto fire or rapid semi-auto fire, and leaves the barrel less prone to gas port erosion. Most AR-15 manufacturers tend to use the 1:9 rifling twist in their barrels, which tends to be able to handle projectiles up to around 69 grain; stabilization problems result if the bullet is any heavier (because they are generally longer and require a faster twist rate). The data package calls for a 1:7 twist in the M16s and M4s, which handles the heavier and longer bullets. As with the bolt, the barrel should be tested with a proof load and MPI tested to check for structural deficiencies. Sights The iron sights on the A1-style AR15s did not have elevation adjustment, only windage. The A2-style sights, as well as many backup iron sights commonly found on many modern ARs, have elevation settings and windage settings; both are adjustable. The standard elevation settings are 300 to 800 meters. Operation The AR-15 utilizes the direct impingement gas operation system, with a rotating bolt breech mechanism. The magazine is loaded into the weapon and the charging handle is racked, pulling the bolt carrier assembly into the receiver extension. The action spring forces the bolt carrier forward, stripping a round out of the magazine and into the chamber. The bolt rotates and locks as it goes into the breech. When the trigger is pulled, the hammer strikes the firing pin, which fires the round. The gas propels the bullet toward the muzzle end of the barrel. As the bullet travels down the bore, the gas produced by the ignition of the propellant fills the gas port (a hole drilled in the barrel that leads into the port in the front sight block, which is connected to the gas tube) and the gas tube, which leads back into the receiver. As the gas is forced into the gas key on the bolt carrier (which is hollow all the way through) via the pressure at the gas port, it forces a rearward motion on the bolt carrier (as well as a forward motion on the bolt), which rotates the bolt, unlocks it, and allows the weapon to cycle. There are several gas systems of differing length, as described below. Carbine-length gas system This system is found on barrels ranging in length from 10.3 inches to 16.5 inches. The gas port is drilled 7.5 inches from the chamber, and the gas pressure typically clocks in at around 26,000 PSI at the port. If there is less barrel after the gas port, there will be less dwell time. Due to this, the gas port on barrels 12.5 inches and shorter are drilled a little wider than it would be on a 14.5 or a 16 inch barrel to compensate for the gas system having less time charged with the pressure necessary to fully cycle the weapon. Due to the high pressures that this system operates with, these weapons are typically much harder on internal parts compared to the other gas systems. Mid-length gas system This gas system is found on either 14.5-inch barrels or 16-inch barrels. The gas port is drilled 9.5 inches from the chamber. As a result, the gas pressure at the port is noticeably less than what would be found on a carbine-length system, clocking in at around 19,750 PSI; however, because there is less barrel after the gas port (therefore allowing less time for the bullet to act as a plug before it exits the barrel) and the pressure is less than a carbine system, the gas port is wider than it would be on a carbine barrel of comparable length. Rifle-length gas system This gas system is typically found on either 18-inch barrels, 20-inch barrels, or 24-inch barrels. The gas port is drilled 13 inches from the chamber. As with the mid-length gas system, the gas pressure is noticeably decreased, clocking in at around 13,500 PSI. The gas port is typically drilled to a diameter of around .090 to .100 inches on these barrels to compensate for the lower pressure, thus allowing for the weapon to cycle reliably. Ammunition and Feeding system This weapon fires both the .223 Remington cartridge and the 5.56 NATO cartridge, depending on the leade in the chamber (if the leade is longer to accommodate the higher pressures generated by the 5.56 NATO cartridge, the rifle will fire both; otherwise, it will only fire .223 Remington ammunition reliably. 5.56 NATO ammunition MUST NOT BE FIRED IN .223 CHAMBERS. Chambers tend to vary among manufacturers). It fires from a detachable magazine, usually with a capacity of 30 rounds. Configurations The rifle comes in many different configurations. The stock is either fixed (the A2 stock is 5/8ths of an inch longer than the A1 stock), or collapsible; there are many different styles of collapsible stocks produced by various manufacturers. The barrel comes in many different lengths, anywhere from 7.5 inches to 24 inches. The length of the barrel tends to dictate the length of the gas system, and the longer the gas system, the easier it is on the more critical parts (such as the bolt). Military Variants M16 First model of the M16. Featured a 20-inch barrel with a 1/12 twist and a non-chrome lined stainless steel bore, an A1 stock, fixed iron sights with rear sights adjustable only for windage, and no forward assist. Early models did not feature a brass deflector. The lower receiver did not feature a rib around the magazine release button. Featured a three-prong flash suppressor. M16A1 First model adopted by the U.S. Army. Featured a 20-inch barrel with 1/12 twist, and a chrome-lined bore. Forward assist added with this model, bird cage flash suppressor also added. Otherwise identical to M16. M16A2 Model adopted by the military around 1986. Features the A2-style lower, the round delta ring, round handguards, the heavy "government profile" 20-inch barrel with 1/7 twist, the A2 pistol grip, a fire control group with SAFE, SEMI, and BURST functions, a longer stock, and iron sights adjustable for windage and elevation (up to 800 meters). M16A3 Same as M16A2, but with automatic fire instead of burst fire. Can also feature a flat-top upper receiver. M16A4 Same as M16A2, but with a flat-top upper receiver, suitable for mounting most optics. M4 This model features a collapsible stock, a carbine-length gas system, shorter handguards, and a 14.5 inch barrel with a notch to accommodate the M203 underbarrel mounted grenade launcher. The FCG has three positions: SAFE, SEMI, and BURST. M4A1 Same as the M4, except for the FCG, which has AUTO in place of BURST. Gallery File:M16A2.jpg|M16A2 File:AR-15CeinerBeltFeedDevice.jpg|M16 rifle with Ceiner belt feed device. File:Colt_m16a2_lmg-1.jpg|a M16A2 LMG References See Also *Armalite AR-10 *Armalite AR-15 *Colt M16A1 *Colt M16A2 *Colt M16A3 *Colt M16A4 *Infantry Automatic Rifle